Gurney Flaps
1. On theoretical background of Gurney flap
Nomenclature Cd = section drag coefficient ; Cl = section lift coefficient ; Cp = pressure coefficient; L = magnitude of lift (downforce); L/D = lift to drag ratio; Rec = Reynolds number based on chord; V0 b c fp Γ ρ = freestream velocity; = span of the foil; = wing chord; = principal frequency; = circulation; = density.
1.1 Overview The Gurney flap is a mechanically simple device consisting of a short strip, fitted perpendicular to the pressure surface along the trailing edge of an airfoil; in particular it is immovable and rigidly mounted by bolts or rivets. It’s just a right-angled plate or an Lshaped steel or aluminium extrusion and, when possible, it is made of carbon fibre. Its first aim was to enhance the spoiler’s downforce in motor racing cars for lateral traction required during high-velocity turns; the racing driver and race car manufacturer Daniel Gurney is generally credited with inventing the device that bears his name. The earliest reference to a Gurney flap was made by Liebeck [1] who carefully studied the spoiler of a race car modified by a 1.25% on chord ( %c afterwards) Gurney flap in wind tunnel; he proposed it changes the Kutta-Joukowski condition on airfoil performing in subsonic condition. Regarding the Kutta-Joukowski condition, as known, when an airflow invests an airfoil, a stagnation point S1 originates and two fluid particles A and B, above and below S1, travel along their respective surfaces at equal speed; see Fig. 1a. Because the upper surface is longer than the lower one, B arrives at the trailing edge ahead of A; it attempts to go around and meets A in S2, the second stagnation point, see Fig. 1b. Kutta-Joukowski condition says that
1
the flow from the upper and lower surfaces must smoothly merge in the wake, then S2 tends to the rear end producing an anticlockwise vortex, see Fig.1c. Then, by The Helmholtz theorem about vorticity, the whole system responds with a clockwise vortex
Nomenclature Cd = section drag coefficient ; Cl = section lift coefficient ; Cp = pressure coefficient; L = magnitude of lift (downforce); L/D = lift to drag ratio; Rec = Reynolds number based on chord; V0 b c fp Γ ρ = freestream velocity; = span of the foil; = wing chord; = principal frequency; = circulation; = density.
1.1 Overview The Gurney flap is a mechanically simple device consisting of a short strip, fitted perpendicular to the pressure surface along the trailing edge of an airfoil; in particular it is immovable and rigidly mounted by bolts or rivets. It’s just a right-angled plate or an Lshaped steel or aluminium extrusion and, when possible, it is made of carbon fibre. Its first aim was to enhance the spoiler’s downforce in motor racing cars for lateral traction required during high-velocity turns; the racing driver and race car manufacturer Daniel Gurney is generally credited with inventing the device that bears his name. The earliest reference to a Gurney flap was made by Liebeck [1] who carefully studied the spoiler of a race car modified by a 1.25% on chord ( %c afterwards) Gurney flap in wind tunnel; he proposed it changes the Kutta-Joukowski condition on airfoil performing in subsonic condition. Regarding the Kutta-Joukowski condition, as known, when an airflow invests an airfoil, a stagnation point S1 originates and two fluid particles A and B, above and below S1, travel along their respective surfaces at equal speed; see Fig. 1a. Because the upper surface is longer than the lower one, B arrives at the trailing edge ahead of A; it attempts to go around and meets A in S2, the second stagnation point, see Fig. 1b. Kutta-Joukowski condition says that
1
the flow from the upper and lower surfaces must smoothly merge in the wake, then S2 tends to the rear end producing an anticlockwise vortex, see Fig.1c. Then, by The Helmholtz theorem about vorticity, the whole system responds with a clockwise vortex